I found this article interesting especial myth #6. Maybe it's how different people see things but it dos fly in the face of most peoples ideas. It has also been written buy a telescope manufacturer which made me wonder, what do you think about these are they myth or not?
Anything added to the light path will degrade the image. Its just a matter of what is visible and acceptable.
A barlow adds extra elements and air/glass surfaces to scatter light. To minimise this effect make sure you get a quality one.
The words "fully multicoated" carry both a price and a performance premium in this regard.